This afternoon, a Japan Airlines 787 flying from Boston to Tokyo was diverted back to Boston after a maintenance alert indicator went off. Initial reports suggest a problem with a fuel pump. But the bigger 787 news comes from London. British investigators are trying to figure out what caused a fire this past Friday on board a Boeing 787. It was parked at London's Heathrow Airport. Today, investigators urged airlines to temporarily disable a device on all 787s called an emergency locator transmitter. The move follows two serious battery incidents earlier this year that prompted a worldwide grounding of the 787 fleet for more than three months.

NPR's Wendy Kaufman is following this story. And, Wendy, first off, the name gives a pretty clear indication of what it is, but why don't you walk us through what an emergency locator transmitter does?

WENDY KAUFMAN, BYLINE: It does exactly what you would think.

BLOCK: Yeah.

KAUFMAN: They're called ELTs for short. They are battery-powered emergency beacons. And they're activated by a pilot or upon impact in the event of a crash. And they help rescue crews find an airplane. Now, the device on the Ethiopian Airlines 787 at Heathrow was made by Honeywell. It is powered by a lithium-manganese battery.

BLOCK: And why are British investigators now focusing on these transmitters?

KAUFMAN: In short, there was a fire and it appears this device was somehow involved.

A bit of background here: The jet had been parked at the airport tarmac for about 10 hours when a very hot fire that produced dense smoke in the cabin erupted. The fire was hottest where this battery-operated ELT and the associated wiring is located, which was near the top of the rear fuselage on the left-hand side. British officials say there is no other system in that part of the plane that contained enough stored power to initiate such a fire if there was no power going to the plane, and ground power had been turned off, though the power cord remained attached.

Having said that, investigators have made it very clear that they still don't know if the fire began with a release of energy from the transmitter's battery or if it was something else, say, an electrical short.

BLOCK: Now, you mentioned that the transmitters are powered by lithium-manganese batteries. But the incidents on the two 787 jets earlier this year involved lithium-ion batteries, not lithium-manganese. What's the difference? What's the distinction there?

KAUFMAN: Well, first off, the battery in the transmitter beacon is much smaller than the lithium-ion batteries that so badly overheated earlier this year. Also, lithium-manganese batteries are generally less volatile than lithium-ion ones. Honeywell, which makes this, of course, says it's produced some 6,000 ELTs of this design. They're on lots of different airplanes. And the company says this is the very first incident of this type ever.

There were, however, much less serious problems with a different version of a Honeywell ELT a few years ago. Those involved improper electrical grounding. So one obvious question for investigators here is: Was there a grounding issue with the 787 device, something that might have led to a short that prompted a fire? Or perhaps, is there something else about the integration of these devices into the 787 that's problematic? Or it's also possible that this incident is just a completely isolated and somehow easily explainable event.

In any case, British authorities made a second recommendation today. They want the Federal Aviation Administration to conduct a safety review of lithium-powered ELTs in other kinds of planes. The FAA says it's reviewing the matter.

BLOCK: Well, how big a deal is this, Wendy, for the airlines flying the 787s and also for Boeing, the company that makes the plane?

KAUFMAN: So far, at least, Boeing says this isn't a big deal. The company says removing the emergency beacon is a simple task. It should take only about a half an hour. As for Boeing's stock, you might think it would be going down through all this stuff. In fact, Boeing is at just about an all-time high. It started the year at $76 a share. It's now at 107.